Nematic chiral liquid crystals (CLCs) are characterized by a helical arrangement of nematic LC molecules. A layer of CLC typically exhibits an optical reflection band due to Bragg reflection in the helical structure. When several layers of CLC are spin-coated and polymerized on top of each other without a barrier layer in between, defect modes can form in their reflection spectrum. By comparing experimental results and simulations, we investigate the origin of the defect modes, thereby revealing details on the behavior of the materials at the interfaces during deposition. Simulations show that these defect modes can originate from the migration of chiral dopant leading to a layer with a smaller pitch or from a discontinuity in the director orientation at the interface between two layers.

Colloidal quantum dots (QDs) are excellent optical gain materials that combine high material gain, a strong absorption of pump light, stability under strong light exposure and a suitability for solution-based processing. The integration of QDs in laser cavities that fully exploit the potential of these emerging optical materials remains, however, a challenge. In this work, we report on a vertical cavity surface emitting laser, which consists of a thin film of QDs embedded between two layers of polymerized chiral liquid crystal. Forward directed, circularly polarized defect mode lasing under nanosecond-pulsed excitation is demonstrated within the photonic band gap of the chiral liquid crystal. Stable and long-term narrow-linewidth lasing of an exfoliated free-standing, flexible film under water is obtained at room temperature. Moreover, we show that the lasing wavelength of this flexible cavity shifts under influence of pressure, strain or temperature. As such, the combination of solution processable and stable inorganic QDs with high chiral liquid crystal reflectivity and effective polymer encapsulation leads to a flexible device with long operational lifetime, that can be immersed in different protic solvents to act as a sensor.

Colloidal quantum dots (QDs) have become an attractive light source for visible photonics. Here, we demonstrate the first integrated LED based on CdSe/CdS QDs, with the emission directly coupled to a silicon nitride waveguide.

We report on the optical analysis of 3D periodic LC nematic structures, using crossed-polarizer transmission microscopy with oblique illumination. The structure is developed in a cell with orthogonally arranged top and bottom photoaligned periodic micro-patterns. We demonstrate experimentally that two unit cell director configurations can be formed in the device. The size of the unit cell is two times larger than the period of the micropattern because symmetry breaking allows avoiding disclinations lines in the bulk of the liquid crystal. As a result, each unit cell can be located in four equivalent positions. Q-tensor finite element simulations show that the two unit cells have different elastic energy. Oblique illumination transmission experiments are in good agreement with numerical simulations. The observed structures are interesting for optical manipulation purposes, liquid crystal based lasing, geometric phase implementation and many other photonics applications.

There are currently two competing technologies on the TV market: liquid crystal TV and OLED TV. Each of them has its strengths and weaknesses, but at the same time, both technologies also see rapid developments that further improve their quality. In an OLED matrix the pixels are driven by a DC current and emit light proportional to the level of the current. In an LCD matrix the pixels are driven by an AC voltage and the polarization of incident light is modified, which, in combination with a polarizer determines the transmission. In both the OLED and the LCD TV red, green and blue color filters are used to render millions of colors by mixing filtered white light. In this presentation we will discuss differences between the two technologies, and address a number of important advances in the field of OLEDs (multilayer stacks, temperature dependency, quantum dot OLEDs) and LCDs (LEDarray illumination, photoluminescent quantum dots, brightness enhancement films, local dimming). In addition a few contribution of the research group will be highlighted [1-4].

We discuss the dynamics of fundamental Gaussian beams launched in saturable and nonlocal nonlinear media. Solely in the presence of a self-focusing saturable nonlinearity, the breathing solitons undergo strong deformation. The addition of a defocusing nonlinearity leads to the generation of couples of solitons. Experimentally, we demonstrate in nematic liquid crystals the formation of multiple spatial solitons starting from a bell-shaped input, with both direction and the number of filaments depending on the input power, confirming the theoretical predictions.

The self-focusing and the formation of spatial solitons attracted a great deal of attention since the early days of nonlinear optics [1]. Essentially a high-power beam changes the refractive index, writing a waveguide capable to confine the beam itself. Accordingly, spatial solitons behave like optical waveguides for beams at different wavelength and low power [2], thus enabling the generation of integrated optical devices written by the light itself. Several all-optical devices have been demonstrated based on this principle, with particular attention devoted in the last two decades to nematic liquid crystals (NLCs) due to their huge nonlinear response [2]. The main nonlinear mechanism in NLC for CW excitations is reorientational, that is, the extraordinary (e-) wave rotates the optical axis of the material, providing a focusing effect [2]. Nonetheless, NLCs are also subject to a strong thermo-optical effect, typically yielding a defocusing effect for the e-wave [3]. Here we demonstrate the excitation of couples of solitons with a single-humped beam caused by the interaction between reorientational and thermal nonlinearity.

Lasing in dye-doped chiral nematic liquid crystal can be realized with low pump energy and relatively high efficiency, thanks to the high reflectivity of the periodic structure. When the helical axis is oriented perpendicular to the substrates, the main lasing peak is normal to the substrates. In some cases, ring lasing of a particular wavelength is observed into an emission cone with axial symmetry. In this paper we explain how scattering of light in the liquid crystal layer leads to optical coupling between normal modes and inclined modes. Based on a numerical model that takes into account spontaneous emission, gain and scattering we show that scattering leads to emission characteristics that are similar to experimental results.